Metal flakes

ABSTRACT

Ultra-fine flakes which have grease-thickening properties are formed by grinding metals in an organic liquid in a vibratory grinding mill having an amplitude of vibration of at least 2mm and a frequency vibration of at least 1,000 cycles per minute.

United States Patent 1191 Groszek 1 July 10, 1973 [5 METAL FLAKES [56] References Cited [75] Inventor: Aleksander Jerzy Groszek, London, UNITED STATES PATENTS England 3,062,457 11/1962 Willems 241/1 [73] Assignee: The British Petroleum Company 3'l00088 8/1963 poqmore at Limited, London, England 3,346,200 10/1967 Lesln et al. 241/175 X [22] Filed: P"- 1971 Primary Examiner-Granville Y. Custer, Jr. [21] Appl. No.: 138,298 Attorney-Morgan, Finnegan, Durham & Pine Related U.S. Application Data [62] Division of Ser. No. 794,757, Jan. 26, 1969, [57] ABSTRACT a ando Ultra-fine flakes which have grease-thickening properties are formed by grinding metals in an organic liquid [52] U.S. Cl. 241/30 in a vibratory grinding mill having an amplitude of vi [51] Int. Cl. B026 19/16 bration of at least 2mm and a frequency vibration of at [58] Field of Search 241/1, 4, 5, 30, least 1,000 cycles per minute.

9 Claims, N0 Drawings METAL FLAKES This is a divisional application of Serial No. 794,757, filed Jan. 26, 1969, now abandoned.

This invention relates to metal powders, in particular, it relates to ultra-fine metal powders which have a high surface area and a low bulk density.

Ultra-fine metal powders have a wide range of applications, and are used, for example, as reinforcing agents for plastics and rubbers, as catalysts, as rocket propellants, as pigments, as fillers, in powder metallurgy, as brazes etc. They are especially useful in forming inorganic and organic based composites and in reinforcing polymeric matrices.

Several methods are known for forming finely divided metal powders including vapour condensation, precipitation from solution, decomposition or reduction of metal salts or oxides, and rapid quenching of molten metal. An effective and commonly used method of metal powder formation makes use of an electric arc. In this method, metal is injected into the anode of a high intensity electric arc and is vapourised by the action of the arc, the vapour is then collected by condensation to form a metal powder.

The metal powders produced by this method are comprised of substantially spherical particles. In some applications of metal powders, flakes are preferable to spherical particles. For example, when an organic or inorganic matrix material is reinforced using metal flakes to form a composite, the overlapping action of the flakes gives rise to a stronger reinforcement than is possible using spherical particles. The overlapping flakes can also form an effective barrier to liquid or gaseous penetration, and such a barrier is very useful in the preparation of high temperature vacuum seals. The overlapping action of the metal flakes in composites also increases the thermal and electrical conductivity of the composite.

It is recognised that mechanical grinding provides, in general a cheaper method for obtaining metal powders than vapour condensation methods or chemical methods. Among known grinding methods are the use of planetary ball mills, and attritor ball mills. However the metal powders produced so far bymechanical grinding have the disadvantages of low surface area and high bulk density.

Bulk density is a useful and convenient method of determining the state of sub-division of a metal in powder form. This measurement can readily be made by placing a known mass of metal powder in a measuring cylinder and agitating the cylinder until the volume of the powder, as measured by the cylinder, remains constant.

The surface area of finely divided metals as determined, for example, by nitrogen absorption, is also a useful measure of the degree of sub-division of the metal poder. However, bulk density and surface area measurements are not exactly correlated. This is thought to be due to various influences, such as particle shape, affecting the two measurements in different ways.

It has been found that metal powders having low bulk densities and high surface areas and energies have useful properties, and suitable metal powders can be produced by grinding in an organic liquid in the presence of an organic grinding acid.

According to the present invention there is provided a metal powder having a bulk density of less than 1 grm per cc, a surface area of at least 1 in per gram. The powder preferably also has a energy of at least 40 ergs per gram.

The invention also provides a method of preparing a metal powder which has a bulk density of less then 1 grm per cc, a surface area of at least 1 m per gram, which comprises grinding a metal in an organic liquid in the presence of a grinding aid in a vibration mill of amplitude of vibration of at least 2 mm and frequency of vibration of at least 1,000 per minute, preferably at least 2,500 per minute.

Preferably the vibration mill is a vibration ball mill through vibration rod mills or other similar vibration mills can be used.

The vibration mill is preferably magnetically driven though it can be mechanically or hydraulically driven.

Preferably the metal powders of the invention have a bulk density of less than 0.5 grm/cc, a surface area of at least 2 square metres per gram and a surface energy preferably of at least ergs per gram.

The surface energy of the metal powders is defined as the heat of adsorption of n-butanol from n-heptane and can be measured using a flow Microcalorimeter as described in Chemistry and Industry Mar. 20, 1965 pages 482-489.

The organic grinding fluids are preferably those liquids distilling below 500 C having a viscosity below 600 centistokes at 100 F and a surface tension below 72 dynes/cm. at 25 C. Preferably the grinding fluid has a viscosity of less than 30 centistokes, more preferably less than 3 centistokes and most preferably less than 1 centistoke at 100 F. Suitable organic liquids are the hydrocarbons such as n-heptane, iso-octane, toluene, hexadecane, cyclohexane and hydrocarbon fractions obtained by the distillation of petroleum. Other organic liquids which can be used include most volatile oxygenhalogen-, nitrogen-, and sulphur-containing liquids, for example, isopropyl alcohol and carbon tetrachloride. Liquids that react with the metal under the condi tions of treatment should of course, not be used.

The method of the invention is applicable to any metal which is capable of being ground, and the preferred metals are aluminum, copper, transition metals such as iron, cobalt, nickel, chromium and molybdenum, and alloys of these metals. The most preferred metals are iron, including sponge iron, cast iron and mild steel, aluminum and brass.

The grinding should be carried out entirely below the surface of the organic liquid, and this is preferably carried out by completely filling the grinding chamber of the vibration mill with the grinding fluid and the organic grinding aid and then adding the metal to be ground. Grinding aids which can be used are those which are known to have a lubricating effect on the unground metal. Copending U.K. application 35119/66 now U.K. Patent No. 1,198445 describes the grinding of metals in an organic load-carrying additive. it has been found that organic lubricating additives which are not recognised load-carrying additives can be added to the organic grinding fluid. Examples of suitable lubricating additives are: the fatty acids and their esters, for example, palmitric, oleic, stearic acid and cetyl alchol and esters, for example the vinyl esters of these acids, e.g. vinyl stearate.

It is well known that the effectiveness of a lubricating additive varies with regard to the metal being lubricated, and it is important that the lubricating additive is chosen with specific regard to the metal to be ground. For example a preferred lubricating additive for aluminum is palmitic acid and for copper is vinyl stearate.

It has been found, surprisingly that the metal particles produced according to the invention have good load-carrying properties. This property will make the powders useful in the manufacture of reinforced, plastic self-lubricated bearing, for example, nylon bearings filled with iron. The metal powders can also be used in the preparation of composites of the type described in co-pending U.l(. application 15535/67 now U.K. Pa-

tent No. 1,224,735. The nature of metal powders also means that they will be very useful in powder metallurgy, for filling plastics in general for example to give a metallic appearance to a plastic article, reinforcing rubbers, and as catalysts etc.

An unexpected property of the powders of the present invention is their partly oleophilic nature and their property of thickening greases.

The amount of finely divided metal powder required to thicken a lubricating base oil to form a grease will depend on the nature of the base oil and the consistency of grease required. For most purposes an amount up to 50 percent wt, based on the final grease, will be used, generally to 40 percent wt.

The lubricating base oil, may be a mineral or synthetic oil. Suitable mineral oils are refined mineral oils obtained from petroleum, for example, those having a viscosity at 210 F within the range from 2 to 50 centistokes, preferably 4 to 40 centistokes.

Synthetic lubricating oils include organic esters, polyglycol ethers, polyphenyl ethers, fluorinated hydrocarbons, silicate esters, silicone oils and mixtures thereof.

The most important class of synthetic oils are the organic liquid polyesters, particularly the neutral polyesters, having a viscosity at 210 F within the range from 1 to centistokes. The expression polyester is used to mean esters having at least two ester linkages per molecule. The expression neutral is used to mean a fully esterified product. Examples of suitable polyesters include liquid diesters of aliphatic dicarboxylic acids and monohydric alcohols (for example, dioctyl sebacate, dinonyl sebacate, octyl nonyl sebacate, and the corresponding azelates and adipates), liquid diesters of aliphatic dicarboxylic acids and phenols (for example, those described in UK. patent specifications 1059955, 1058906, 1044550, 1044883 and in copending U.1(. patent application 31249/65 now U.K. patent specification No. 1,105,965), and more complex polyesters (for example, those described in UK. patent specifications 666697, 743571, 780034, 861962, 933721, 971901, and 986068 and in co-pending U.K. applications 38165/64 and 31249/65, now UK. Patent Specification Nos. 1,129,965 and 1,105,965, respectively).

The finely divided metal powder can be incorporated into a grease by a number of methods. It is preferred to incorporate the finely divided metal powder into a grease, immediately after grinding. However, if the finely divided metal powder is prepared some time before incorporation into the grease, it is preferred to store the finely divided metal powder in an air-tight container to prevent deterioration.

The slurry of finely divided metal powder can be converted into a grease by, for example:

a. The grinding fluid is filtered off. The resulting filter cake is ground by, for example, feeding the cake through a colloid mill and stirring the resulting powder into the oil. The resulting grease is finished by colloid milling.

b. The grinding fluid is boiled off rapidly to avoid the formation of a metal powder cake and the resulting powder is stirred into the oil and the grease finished by colloid milling.

0. Oil is added to the slurry of finely divided metal powder and the grinding fluid distilled off.

d. Oil is added to the slurry of finely divided metal powder and the mixture circulated through a homogeniser (for example, of the Manton-Gaulin type) so that temperatures up to or exceeding 140 C are produced. The temperature must be high enough to drive off the grinding fluid.

e. The metal might also be ground directly in the base oil for the grease. For example, a low boiling point, low viscosity, low surface tension mineral lubricating oil with a viscosity up to 600 centistrokes at F (38 C) can be used. Elevated temperatures up to 400 C can be used during the grinding.

Methods (c), (d) and (e) are particularly preferred. In general, the finely divided metal powders can be incorporated into the base oil either at ambient temperatures or, ifdesired, at elevated temperatures, for example, up to 400 C.

The greases according to the invention have remarkably high Drop Points. When their drop points are measured according to the 11 or ASTM standard methods, they are found to be above 400 C; such greases are described as infusible and are difficult to produce by conventional methods. By using carefully selected base oils, for example, synthetic oils with high oxidation and thennal stability, greases having a unique combination of properties can be produced.

In certain circumstances it may be advantageous to add dispersants to the metal either before grinding or after grinding. In this way the dispersion of the finely divided metal powders may be aided. Viscosity index improvers, metal deactivators, anti-corrosion agents anti-oxidants etc. can also be added to the greases according to the invention.

The metal powders can also be used as reinforcing agents for plastics materials and elastomers. They can also be used in paints and fuels. I f

Metal powders produced according to the invention are further described in the Example.

EXAMPLE Various metals were ground in n-heptane containing various lubricating additives using a Megapact mill manufactured by Pilamec Limited.

In this mill the grinding chambers are steel cylinders of 1% inches internal diameter by 15 inches long and are nearly filled with /4 inch diameter hardened steel balls. The mill is fitted with a V4 horsepower electric motor and the oscillation can be adjusted from 1 to 4 mm. In operation, each cylinder was filled completely with n-heptane, containing dissolved therein the lubricating additive and steel balls and from 10 to 50 grams of metal powder of from 50 to 400 British Standard mesh were added. The ends were then sealed with metal caps fitted with rubber washers and grinding carried out at an oscillation of 4 mm and a frequency of 3,000 vibrations per minute. After grinding the balls 5 6 were sieved from the slurry of metal powder and n- 4. A method as claimed in claim 3 in which the orhcptanc and lubricating additive and the treated metal ganic grinding aid is a fatty acid, a fatty acid ester or a powder recovered by filtration, washing and drying. fatty alcohol, Their properties are shown in the Table. 5. A method as claimed in claim 4 in which the fatty TABLE I Surface Grind- Amount, area, Grease forum llxperimrmt int: time, Additive in percent of Bulk density, sq. 111. tion with 0. Metal hours n-heptitne n-heptnne grins/cc. per grm. mineral oil Vinyl Stetrliifl 6 12 Forms grease.

13 D0. 11 D0. 12 De. 7 Do.

Do. Do. Do. Do.

What we claim is: acid is palmitic, oleic or stearic acid. 1. A method of preparing a metal powder having a 6. A method as claimed in claim 4 in which the fatty bulk density of less than 1 gram per cc and a surface acid ester is the vinyl ester. area of at least l square metre pre gram which com- 7. A method as claimed in claim 4 in which the viprises grinding a metal in an organic liquid in a vibrabrating mill is a vibration ball mill. tion mill of amplitude at least 2 mm and of frequency 8. A method as claimed in claim 1 in which the metal of vibration of at least 1,000 cycles per minute in the is aluminum, copper, a transition metal, or an alloy of presence of an organic grinding aid. any of these metals.

2. A method as claimed in claim 1 in which the vibra- 9. A method as claimed in claim 8 in which the transition mill has an amplitude of at least 3 mm. tion metal is iron, cobalt, chromium, nickel, or molyb- 3. A method as claimed in claim 1 in which the fredenum. quency of vibration is at least 2,500 cycles per minute.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,744,726 Dated October 2, 1973 Patent No. lnvntofls) Aleksander Jerzy Groszek It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Foreign Agglication Priority Date I Column 2, line 63, "palmitric" should read palmitio Signed and sealed this 27th day of August 1974.

(SEAL) Attest: I

C. MARSHALL DANN MCCOY M. GIBSON, JR.

Commissioner of Patents Attesting Officer USCOMM-DC 60376-P69 R 0.5. sovznuuzm PRINTING omc: an 0-.

FORM PO-IOSO (10-69) 

2. A method as claimed in claim 1 in which the vibration mill has an amplitude of at least 3 mm.
 3. A method as claimed in claim 1 in which the frequency of vibration is at least 2,500 cycles per minute.
 4. A method as claimed in claim 3 in which the organic grinding aid is a fatty acid, a fatty acid ester or a fatty alcohol.
 5. A method as claimed in claim 4 in which the fatty acid is palmitic, oleic or stearic acid.
 6. A method as claimed in claim 4 in which the fatty acid ester is the vinyl ester.
 7. A method as claimed in claim 4 in which the vibrating mill is a vibration ball mill.
 8. A method as claimed in claim 1 in which the metAl is aluminum, copper, a transition metal, or an alloy of any of these metals.
 9. A method as claimed in claim 8 in which the transition metal is iron, cobalt, chromium, nickel, or molybdenum. 